Process of stabilizing autocatalytic copper plating solutions



grouping and R United States Patent PROCESS OF STABILIZING AUTOCATALYTICCOPPER PLATING SOLUTIONS Maynard C. Agens, Schenectady, N.Y., assignorto General Electric Company, a corporation of New York The presentinvention relates to a process for stabilizing copper plating solutionsused to autacatalytiaclly plate copper onto metals or onto non-metallicsurfaces. More particularly, this invention relates to a stabilizationprocess which comprises producing oxidizing conditions within saidplating solutions whereby the formation of cuprous oxide, which causesthese solutions to become unstable, is greatly minimized orsubstantially prevented without the prevention of the reducing reactionwhich causes the copper to plate.

In my copending application S.N. 725,449, the copending applications ofRobert M. Lukes, S.N. 725,450 and S.N. 725,452, all of which are filedconcurrently herewith and assigned to the same assignee as the presentinvention, in the patent of A. E. Cahill et al., US. 2,874,- 072 andassigned to the same assignee as the present invention, and in thephamphlet Wein, Samuel, Copper Films, PB 111, 237, US. Department ofCommerce, Oflice of Technical Services, 1953, there are disclosedplating solutions comprising aqueous, alkaline solutions containingformaldehyde and cupric ion complexed to prevent precipitation of cuprichydroxide to which my process is applicable.

Specifically, Cahill discloses complexing agents which are tartrates andsalicylates used in the presence of carbonates all of which are watersoluble. Lukes discloses in his application S.N. 725,450 complexingagents which are ethyleneaminoacetic acids which are selected from theclass consisting of ethylenediaminetetraacetic acid,diethylenetriaminepentaacetic acid and1,2-cyclohexylenediaminetetraacetic acid. In application S.N. 725,452,

Lukes discloses complexing agents corresponding to the formula:

$I CHzCH-OH),,

om-o 0 on where m and n are both integers and are at least 1 and notmore than 2, p is an integer and is at least 0 and not more than 1, thesum of m, n and p equaling 3;.R is a member of the group consisting ofhydrocarbon radicals having from 1 to carbon atoms, and the is a memberof the group consisting of hydrogen and methyl. In my copendingapplication I disclose complexing agents which correspond to theformula:

BI Cardin-0H "ice where R is a hydrocarbon radical having from 1 to 10carbon atoms and, in addition, when p=0 and n=1, R also represents theR! CHr-H-OH CH2CH2-N GHa-(fH-OH RI grouping. R is a substituent selectedfrom the group consisting of hydrogen and methyl, m is an integer and isat least 1 and no more than 4, n is an integer and is at least 0 and nomore than 3, p is an integer and is at least 0 and not more than 1. Theparticular values of m, n, and p must be so chosen that they fulfill thefollowing equation: +p

Further details concerning the use of these solutions are shown by theexamples and by reference to the Cahill patent and the above-mentionedcopending applications.

Many of the solutions disclosed in the Wein publication can form copperfilms or mirrors on surfaces that have not been sensitized with metallicfilms.

All of these solutions autocatalytically plate copper. By thisterminology I mean that the solutions are capable of plating copperwithout the application of an external force, e.g., electricity. 7

Solutions known to autogenously plate copper as a smooth, adherentcoating do so according to the following equation:

A subordinate and yet significant, reaction also occurs as illustratedby the following equation:

Since the reaction expressed by Equation II does not require activationby an extraneous material, it will occur merely upon the solutionstanding at room temperature and is hastened by heating. The cuprousoxide product is reduced by the alkaline formaldehyde solution to coppermetal which is one of the metals that is catalytically active ininitiating the plating reaction expressed by Equation 1. Therefore, thereaction of Equation Ilcauses these solutions to spontaneously decomposeand'to become useless upon standing for long periods. j Although thecomplexing agents used in any of the above copending applicationsgreatly suppress the reaction expressed by Equation 11, nevertheless,high concentrations of cupric ion andthe high alkalinity of thesolutions over-ride some of the efiect of these complexing agents.Furthermore, it is not possible to completely suppress Equation H withany complexing agent.

Therefore, all of the solutions used to autocatalytically plate copper,such as those described in the above-identified applications have afinite life, that would be very desirable to extend. I have now found avery simple and easy method for performing this function. My inventioncomprises aerating these solutions with an oxygen'containing gas duringstorage or other periods of nonuse, and also while it is being used toplate copper. Aeration can be accomplish with pure oxygen or oxygendiluted with any inert gas such as argon, neon, krypton, nitrogen, andthe like,'the simplest gaseous mixture to use being air itself eventhough it does contain a minor amount of carbon dioxide which reactswith the alkali in the solution. From what I can determine byexperiment, the effect of oxygen is to either prevent the formation ofcuprous oxide or to reoxidize the cuprous oxide back to soluble cupriccompounds so quickly that no noticeable oir will usually be required..A'filter i 3 precipitate can be observed. This is based on myobservations that the plating solutions remain clear and free of anyprecipitate for extended periods of time providing a. gasv containingelemental oxygen, more commonly re ferredcto as an oxygen-containinggas, is blowing through the solutions. If I stop the flow of gas, aprecipitate will form in time which will disappear almost as soon as Istart aerating again. However, if I delay the start-up of the gas streamfor a period long enough that the cuprous oxide is converted to copper,the copper will not dissolve. To restore stability in this case, thecopper must be removed from contact with the solution, for example byfiltration, centrifugation, decantation, etc. Furthermore, I have foundthat best results are obtained Ijhave the gas dispersed a s'very finebubbles which areuuniformly distributed throughout the entire solution.Should, I fail to do this, I have noticed'that precipitation will occurin those-portions of the'solution not contacted by the gas, stream.Immediate correction of this condition can be effected by vigorousagitation. I have found that the best method of carrying my inventioninto effect is 'tointroduce the oxygen containing gas stream through adisperser such as a fritted glass disk or a po-' rous. ceramic disk sothat the gas is. broken up into very fine bubbles, and to use vigorousagitation to insure that these bubbles are distributed throughout'theentire volume of the plating, solution. Sinceall metals that arenotattacked by the alkaline. solutions. are catalytically active withthe plating solution, the gas inlet and disperser must'be non-metals.Alternatively, of course, more than one stream of gas can be introducedinto the solution; Providing the gas inlets are properly placed, and thevolume oftheincoming gas is sufiicient, no further agitation is requiredsince the gas streams themselves will provide suflicient agitation ofthe solution. Preferably, the entire liquid phase should be saturatedwith theoxygencontaining gas at all times. Filters can be used inconjunction with the aeration to' remove any solid material.Superatmospher-ic gas pressure can be'used but is notrequired.Sub-atmospheric pressure is not desirable since it does not permit aselfective saturation of the solution with the oxygen-containinggas as dohigher pressures. 1

. I have also found that aeration can be accomplished by flowing thinfilms of the plating solutions over the surfaces to be plated. By thismeans, a small volume of l-iquidis exposed to a large volume ofoxygen-containing gas. .By inclining the surfaces so that the liquidflows rapidly, agitation is also provided. The surfaces to be plated canbe arranged in a stepwise fashion so that the liquid cascades from onesurface to the nextinseries. If non-metallic'spray equipment is used,the plating solu tion can be sprayed as'anatomized 'm'istv to provideaera tion. In anyof these applications, if a reservoir ofliquid ispresent, additional. aeration of the liquid in the rese'r can; also beinstalled 1 1112118 line to remove any solid material; 1

Since theamount of oxygen actually consumed is extremely small, there isno critical concentration of oxy-- gen required in the gas compositionused in my process. The'actual amount of oxygen is dependent on thecupric ionooncentration. This amount can be supplied by low oxygenconcentration at fast flow rates or highconcem tration at low flowrates. From a practical standpoint,- there is no advantage in using agas containing less oxygen than the approximately 20% contained in air,since this is the cheapest and most readily available oxygen-containinggas. Oxygen in a compressed gas cyclinder is a convenient source to usewhere there is no source of compressed air, or where mobility of theplating equipment is desired; However, my results with pure oxygen haveshown no added benefits over the results I obtain with Theoxygen-containing gas should be free of oils,

greases and. other contaminants that would afiect'the quality of the.copper'plate produced by the solutions;

Preferably, the container for the plating solutions should s r am 4 be 9arreassd a he e t gases o n t a -T a ma t than the amount of reagents.In this regard, some loss of formaldehyde will always occur, due to itsvolatilization in the gas stream. This loss can be compensated for byadditions of aqueous formaldehyde continuously or at prescribedintervals so as to maintain the concentration required to produce theplating reaction. In order, that those skilled in the art may understandhow my invention can be carried into eifect, the following examples aregiven by way of illustration, and not by way of limitation.

Example 1 One liter of a plating solution was made by dissolving thefollowing ingredients in enough water to make one liter of solution:

Aqueous 37% formaldehyde,.25 m1.

Theabove solution was divided into. two portions; one portion was usedasa control and was allowed to standat room'temperature without anyfurther treatment. The other portion was aerated by introduction of 'airinto: the solution through a fritted glass disk. It was not mechanicallystirred. At the end of approximately two hours, the controlhadst'arfed'to decompose, as evidenced by the precipitationofi'copper.The aerated sample remained clear and unchanged. After leaving thesolutions overnight; the control .sample had completely decomposed, asevidenced by the solution being colorless and all of the copper being aprecipitate in the bottom of the'contain'er. The aerated sample stillblue in color and had apH of 11.5,; There was a smali' amou'nt of coppermetal pres.- entin the'solution indicatingrthat the solution 'had notbeen well stirred. The example was repeated, except thatthistimethesolution was stirred vigorously in addition tobein'gfareated.At the endof "=12 hours, he decompoe sition of the plating solution hadoccurred, thepH was 119. A pieceof'sand-bl-asted paper-base,phenolic-resin laminatewas sensitized by immersing it for one minute ina stannous chloride solution 'made by dissolving l0 gra'rns ofstannouschlon'de and 10 milliliters of 12 molar hydrochloricacid in oneliter of water, rinsed with water, dipped for oneminute into. a'palladium chloride solution made by 'dissolvingdigram of palladiumchloride and IOmillilite'rs of'l2- molar'hydrochloric' acid in one literof water, andgagain rinsed in water. 'When the sensitized laminate wasplaced in the aerated solution, a coherent, bright film of copper platedonto the laminate simultaneously with the evolution of hydrogen from thesurface.

Example 2 The solution as described in. Example 1 was duplicared d vid dim worortio n n p io a of nitrogen was adniitted atthe rate of 0.4 cubicfoot per home; In the-second port-ion, a flow of pure en Wfl5 4mi s1et-t a o 0-4 c t p hour. At the end of 1- 6; hours, thesolution aeratedwith the nitrogen was beginning to decompose and at the end of 2 /2hours, was completely decomposed, as'evidenced by the solutionbeingcolorless; The solution aerated with oxygen was still bright blue incolor, with no signs of decomposition at the end of 48 hours, but therehad been some depletion of formaldehyde. aldehyde; lost was replacedand,thepH adjusted to 128. When a piece of paper-base, phenolic-resinlaminate which had beensensitized 'as in Examplel was placed in thissolution, it was found that the'copper plated on the sensitized surfaceat a rate of'about 1 mil per hour. When 4 mils of copper had built, up'on the board, it was removed from the solution. 7 The solution wasaerated with oxygen for an additional 24 hours." When a sensitized pieceof phenolic-resin laminate was inserted into the The amount offormplating solution, the copper deposited. at' the same fast rate of 1mil per hour. After a film of 3 mils of copper had been plated onto thesample,.it was removed. Altogether, the solution hadremained clear anduseful as a plating bath for about 80 hours with no signs ofdecomposition.

Example 3 Two liters of solution were prepared containing the followingingredients in the stated concentrations:

Molarity Cupric sulfate 0.10 Ethylenediaminetetraacetic acid 0.10Potassium hydroxide 0.80 Formaldehyde 0.30

A piece of paper-base, phenolic-resin laminate having an area of 50 sq.inches which had been sensitized with palladium as in Example '1 wasplaced in this solution. Air was bubbled in through a fritted glassfilter diifuser at the rate of 0.2 liter per minute, while the solutionwas stirred vigorously with a mechanical stirrer. At the end of 5 hoursan additional 25'milliliters of formaldehyde was added and 2 millilitersof a wetting agent comprising an ethylene oxide and propylene oxidepolymer was added. The thickness of the copper coating was 0.6 to 0.7mil thick and no decomposition of the plating solution had occurred. Atthe end of an additional 1 /2 hours, the solution was platingsatisfactorily and was left overnight, for 15 hours. No decomposition ofthe solution occurred during this time and the solution was stillplating, however, at a very slow rate. An additional 50 milliliters offormaldehyde was added and enough potassium hydroxide was added toincrease the initial molarity of the base by an additional 0.80. Thiscaused the solution to start plating rapidly again. At the end of 4 /2hours, the solution was still plating at a rate of about 0.1 mil perhour and the total thickness of the copper plate was about 2 /2 mils.Although no undesirable decomposition of the solution occurred, theplating reaction was stopped at this point by acidifying the solution toa pH of 2, whereby 56 of the original 60 grams of theethylenediaminetetraacetic acid was recovered by filtration of thesolution, since this reagent is insoluble in the reaction mixture atthis pH.

Example 4 One liter of solution was made containing the followingingredients:

Grams Cupric nitrate trihydrate 15 Sodium hydroxide 20 Sodiumbicarbonate Sodium potassium tartrate 30 This solution was divided intofour equal portions of 250 milliliters each, and into each portion 25milliliters of 37% aqueous formaldehyde was added. One portion was leftstanding at room temperature as a blank; the second portion was aeratedwith air using a fritted glass disk to disperse the gas stream. Apaper-base, phenolicresin laminate which had been sensitized as inExample 1 was placed in each of the third and fourth portions. The thirdportion was not aerated and the fourth portion was aerated in the sameway as the second portion. In both the third and fourth solutions, theseeded board was plated rapidly with copper. At the end of about 20minutes, both the first and third solutions which had not been aerated,were starting to decompose as was evidenced by the formation ofprecipitate of cup'rous oxide. Both the second and fourth solutions werestill clear, with no evidence of decomposition at the end of 7 hours.

Example 5 A plating composition comprising three solutions useful inmaking copper mirrors was made up as follows.

Solution A:

Cupric sulfate pentahydrate ..g 20 Glycerol ml Aqueous ammonia (28%) ml20 Solution B: p p

' Sodium hydroxide (9% aqueous sol.) ml 400 Sucrose (10% aqueous sol.)ml 200 m1... 0.5 Water ml 250 Solution C:

Aqueous formaldehyde (37%) ml 80 Water ml 1250 The three solutionscombined in the proportion Solution A ml Solution B ml 850 Solution C ml1330 The resulting plating solution was divided into two portions. Oneportion was aerated with air at the rate of 0.5 cubic foot per hourwhile the other portion was permitted to stand at ambient temperature asa control. When a piece of paper base-phenolic-resin laminate that hadbeen sensitized with palladium as in Example 1 was placed in each of thetwo portions copper plated onto the sensitized area. The pieces oflaminate were removed and the solutions allowed to stand overnight. Inthe morning the non-aerated sample had formed a smooth mirror over theentire area of the glass beaker contacted by the solution which was nowdepleted of copper. No sensitization of the non-metallic (glass) surfaceis necessary with this solution to obtain the mirror. The aerated samplehad not decomposed either by forming a copper mirror or by precipitatingcuprous oxide. At the end of three days it was still stable and platedcopper on the sensitized laminate. By stopping the aeration of a sampleof this solution, a copper mirror will form on the walls of thecontainer on standing overnight.

7 Example 6 One liter of solution was made containing the followingconcentration of ingredients:

Molarity Cupric sulfate 0.1 Glycerol 0.45 Ammonium hydroxide 0.4 Sodiumhydroxide 0.6

Formaldehyde (37% aqueous sol.), 80 ml.

Example 7 One liter of solution was made containing the follow ingingredients in the stated concentration:

Molarity Cupric sulfate 0.1 Glycerol 0.45 Sucrose .15 Ammonium hydroxide0.4 Sodium hydroxide 0.4

Aqueous formaldehyde (37%), 80 ml.

The procedure of Example 6 was repeated with almost identical results.The test for the aerated sample was discontinued at the end of eighthours with no sign of decomposition.

Example 8 In order to obtain some explanation as to the function ofoxygen in preventing the decomposition of these platr 7 ing solutions, asolution was made up containing the following ingredients in the statedconcentrations:

. I a r Molarity Ethylenediaminetetraacctic acid 0.10 Potassiumhydroxide 0.60

into this solution, 0.05 mole of-:cuprous oxide was added, mechanicalagitation was, supplied, and air at the 1 The following example wascarried out to see whether I or not aeration'could stop decomposition.once it had been initiated. One liter of solution was made up containingthe following ingredientsi Grams Cupric sulfate pentahydrate 25Ethylenediaminetetraacetic acid 60 Sodium hydroxide 52 Twenty-twomilliliters of 37% aqueous formaldehyde solution was added to 875milliliters of the above solution, which was agitated with a mechanicalstirrer and aerated with oxygen at the rate of 0.4 cubic foot per hour.A

sample of this solution showed a light transmission of 98 /2 whenmeasured in a spectrophotometer using lightof a wavelength of 450millimicrons. The mechanical stirrer was shut off and the stream ofoxygen stopped. At the end of approximately 1 hour, the solution becameturbid, indicating that it was starting to decompose. At

c this point, the light transmission was found to .have

dropped to approximately 2%, n The stream of oxygen and the stirringwere againstarted, which immediately caused the turbidity to disappear.After 15 minutes of aeration, a sample was taken and it was found thatthe light transmission had increasedto 90%. During the entire experimentthe pH of the solution had remained at 12.8. This example shows thataeration with an oxygen- 7 containing gas can stop deednipositienofthese plating solutions if initiated in the early stages'of the,decomposition. i

As illustrated by the above examples, my method of stabilization isapplicable to all plating solutions which are based on the reactionofformaldehyde. with a cupric salt complex in an alkaline solution :to.producea film of' copper metal. Thisrstabilizationv is apparently causedby the abilitylof the oxygen-containing gas to prevent the precipitationor, formation ofic'uprous oxide by thefsec ondary reaction outlinedabove.

The above examples haveillustrated many o'f the modifications andvariations of the present invention, but obviously, other modificationsand variations are possible in light of the above teaching. For example,the plating solutions aerated according to my invention can be used athigher temperatures than the non-aerated plating solutions. It istherefore to be understood thatchanges'may be made in the particularembodiments of. the invention described which are within the fullintended scope of'the invention as defined by the appended claims.

What I claim as new and desire to secure byLetters Patentof the UnitedStates is:

1. The process for stabilizing aqueous, alkaline,formaldehyde-containing copper plating] solutions used'toautocatalytically plate copper onto metals or non-metallic surfaceswhich comprises aerating such solutions with a gas containing elementaloxygen.

.2. The process of claim 1 wherein the gas is air.

3. The process of claim 1 wherein the-gas is substantially pure oxygen.

4. The process of claim 1 wherein the plating solutions are saturatedwith the gas. i

v 5. The process of claim 1 wherein aeration is used in conjunction withmechanical'agitation.

6. The process of claim 1 wherein the gas is dispersed throughout theentire volume of plating solution.

7. A solution for autocataly'tically plating copper which is stabilizedagainst self-decomposition comprising an aqueous, alkaline solution offormaldehyde and a cupric ion which has been complexed so that thesolution is free of cupric hydroxide, said solution containing a gascontaining elemental oxygen.

8. The solution of claim.7 wherein the gas is air.

9. The solution of claim 7 wherein the gas is substantially pure oxygen.a

10. The solution of claim 7 wherein the solution is saturated with thegas.

References Cited in the file of this patent Owen Aug. 6, 1957

7. A SOLUTION OF AUTOCATALYTICALLY PLATING COPPER WHICH IS STABILIZED AGAINST SELF-DECOMPOSITION COMPRISING AN AQUEOUS, ALKALINE SOLUTION OF FORMALDEHYDE AND A CUPRIC ION WHICH HAS BEEN COMPLEXED SO THAT THE SOLUTION IS FREE OF CUPRIC HYDROXIDE, SAID SOLUTION CONTAINING A GAS CONTAINING ELEMENTAL OXYGEN. 